Using scroll type plates to compress fluids has been known since ancient time. It is not until 1886 that a scroll type fluid treating apparatus was developed. An early patent (U.S. Pat. No. 801,182, "Rotary Engine") issued in 1905 to Leon Creux discloses this general type of device. Later in 1970, U.S. Pat. No. 3,884,599 provides the base for commercializing rotary fluid machines of this kind. Study and research have since significantly devoted to the development of the kind of machine.
The compression mechanism of the scroll type compressing apparatus comprises at least a housing or a frame inside which a stationary scroll is fixed, a crankshaft driven by a rotary motion driving means, such as an electrical motor, to drive an orbiting scroll which orbits around the center of the stationary scroll wrap, and an Oldham-coupling ring interposed between the frame and the orbiting scroll member to serve as an anti-rotation coupling means. Both scrolls are constituted by an end plate on which an involute wrap plate is normally fixed. The involute wraps of both scrolls have the same pitch so that when the wrap plates of the scrolls face-to-face inter-engage with each other, a sealed chamber is formed by the end plates and the wrap plates of the scrolls.
As described in U.S. Pat. No. 3,884,559 issued to young et al., the scroll type apparatus operates by moving the sealed chamber inside which fluid to be compressed is hermetically contained from one region to another region which may be at a different pressure If the fluid is moved from a lower to higher pressure region, the apparatus is serving as a compressor. If the movement is from the higher pressure region to the lower pressure region, then it is an expander. The movement of the sealed pocket of fluid is achieved by the orbiting motion of one scroll member around the center of the other scroll member. When the movement is in such a direction as to have the volume sealed chamber gradually reduced, the pressure of the fluid contained therein increases.
The conventional scroll type compressing apparatus suffers the problems of sealing and wearing which in general limit the efficiency of the apparatus. The sealing problem comes from that when the pressure of the sealed fluid increases, an increasingly great resultant force thereof acts upon the scrolls, intending to separate these scrolls apart and thus increasing the gap therebetween. Under the circumstances, the fluid contained in the sealed chamber will not be completely sealed.
The wearing problem comes from the sealing problem. To overcome the sealing problem, an external force which is at least greater than the largest resultant force of the fluid pressure is applied to the scrolls to hold them together. This results in a frictional contact between the scrolls and thus when the machine operates, the frictional contact between fixed parts and moving parts will result in wear therebetween.
Among the patents regarding scroll type compressor, U.S. Pat. No. 4,846,639 teaches the use of springs or external pressure source to generate the external force used to hold the scrolls together. As mentioned in the previous paragraph, such an external force results in a great frictional contact which requires a great start-up torque to start the machine.
Another way to overcome the sealing problem is disclosed in the Japanese Laid-open patent Application No. 55-46081 wherein high precision machined parts are assembled in a high precision way to form very small gaps between the scrolls. Such small gaps are filled with lubricant which forms thin films therein to block the gaps through which the compressed and thus pressurized fluid may escape. The disadvantage is that when the fluid is compressed and pressurized, its temperature will rise and during a long term operation, the temperature increase accumulated in the scrolls may be significant. In consequence, thermal expansion of the scrolls is inevitable and it may sometimes completely fill the gap between the scrolls. In that case, wearing occurs with the consequence of increase of driving torque and eventually burnout of the scrolls.
A further way to solve the sealing problem is to use tip sealing means. Creux (U.S. Pat. No. 801,182) has already taught using tip seal to overcome the sealing problem. Among the patents regarding scroll type compressor, U.S. Pat. Nos. 4,564,343, 4,740,143 and 4,864,639 and Japanese published patent Application Nos. 51-117304 and 57-180182 all teach the use of tip seal(s), although different in design, to seal the gap between the scrolls. Taiwanese Utility Model patent Application No. 77206560 also discloses a different design of the tip seal for scroll type compressor. The disadvantage of using tip seal is the increase of frictional drag force and the wear of the seal. Sometimes, it is also required high machining precision in installing tip seal.
Using the high pressure fluid generated by the compressor itself is another way to overcome the sealing problem. This design is disclosed in U.S. Pat. Nos. 3,600,114 and 4,365,941 and Japanese Laid-open patent Application Nos. 62-18758 and 62-37238. This is usually done by transferring high pressure fluid, which may be a fluid different from the fluid compressed by the movement of the scrolls, into a back pressure chamber to generate a force acting upon the orbiting scroll to move the orbiting scroll toward the stationary scroll and thus reducing the gap dimension therebetween. The disadvantage is that continuously conducting high pressure fluid to the back pressure chamber will certainly sacrifice volume efficiency of the compressor. Further, since the variation of pressure during the compression cycle will affect the stability of the back pressure and thus an over-pressurized model must be adopted to ensure a sufficient sealing force, an unnecessary friction is presented between the scrolls as a result. This results in a waste of energy.
It is therefore desirable to have a back pressurized self-sealing scroll type compressor wherein the pressure of the compressed fluid is transferred into the back pressure chamber to maintain the hermetical contact between the scrolls and thus eliminating leakage of the compressed fluid therethrough while a gap of a suitable dimension is maintained between the scrolls to eliminate frictional contact therebetween, a thick fluid, preferably the lubricant used to lubricate the compressor, being filled the gap between the scrolls to form a film blocking the gap to further seal the fluid that is being compressed from leaking through the gap